The present invention relates generally to the field of instruments for measuring and analyzing the optical properties of organic materials and, more particularly, to improved optical systems for measuring and analyzing optical densities of organic materials at various wavelengths to determine the percentage of certain constituents contained in the test material.
During the past few years there has been an increasing demand by the agricultural community for versatile test instruments capable of rapidly determining moisture, oil and protein content in produce, grain and other agricultural products. The traditional analytical laboratory techniques, such as the Kjeldahl technique for measuring protein, are extremely accurate but require the services of a skilled chemist. The results, furthermore, are not immediately or readily available. Buyers of agricultural products have demonstrated an increasing interest in accurate and rapid determination of moisture, protein and oil percentages of the various products purchased. The wheat export market, for example, has seen the widespread introduction of selling on the basis of guaranteed protein content. This competitive pressure has increased the requirement for the commodity handler, from the country elevator to the export terminal, to rapidly and accurately sort grains and other products by their protein percentages, as well as by oil and water content, where applicable. The need for versatile, yet low cost, advanced equipment, which combines and improves upon recent scientific findings in the field of non-destructive testing of agricultural products has greatly increased. For the maximum usefulness of commodity handlers, such an instrument must not place high demands on the skillfulness of the operator or require a specialized knowledge of the scientific basis for the end result.
Recent developments have provided instruments which are able to satisfy some of the above requirements of commodity handlers. The optical analyzer described by Eugene R. Ganssle and Donald R. Webster in U.S. Pat. No. 3,765,775, assigned to the assignee of the present application, provides an optical system for measuring the optical density of an object such as a grain sample which is illuminated with light of different wavelengths in a continuous sequence. The specimen sample is illuminated with light sequentially filtered by a continuously rotating disc carrying a plurality of narrow band optical interference filters. The combined output of several photodetectors positioned to receive light transmitted through, or, alternatively, reflected by the specimens is selectively sampled after passing through a logarithmic amplifier to obtain readings at two discrete wavelengths which are then compared in a differential amplifier to provide the required measurements. Although the system described in U.S. Pat. No. 3,765,775 is satisfactory for its intended purpose, its ability to make readings at various wavelengths is naturally limited by the number of filters carried by the rotating disc. It is, therefore, not possible to take readings at wavelengths between those of two adjacent filters.
The disadvantages of the above described system have been mitigated in a more recent optical analyzer described by Donald R. Webster in U.S. Pat. No. 3,861,788, also assigned to the assignee of the present application. The instrument described therein provides an automatic test apparatus for gauging the percentage of various constituents in organic substances by comparing the reflective optical density of the sample at various wavelengths. This device contains narrow band optical filters connected together in the form of a rotatable paddlewheel positioned so that the filters can be individually swept through the incident light path between the specimen and a wide band light source. As the filter wheel turns, the band of light passed by each filter is progressively shifted with the changing angle of the filter relative to the light path. The paddlewheel filter configuration includes opaque vanes extending from the ends of the filters to peiodically interrupt the passage of light to the specimen. Photodetectors are positioned to sense the level of light reflected from the specimen. The output of the photodetectors is sampled at predetermined times relative to the rotation of the filter wheel to yield values indicative of the reflected intensity at certain wavelengths. An electronic circuit utilizes this data to calculate three optical density difference values corresponding to moisture, protein and oil contents of the specimen sample. The difference values are automatically inserted in three linear equations which are solved to obtain readings representing the three percentages of oil, water and protein content of the specimen. Each time a new specimen is loaded for testing, the instrument is automatically calibrated against a standard sample, preferably Teflon (trademark). The output of the photodetectors is sampled in a special circuit which subtracts the level of dark period current from the output when the photodetectors are illuminated.
Yet another recent prior art photo-optical technique for determining, for example, the fat content of meat is described by George F. Button and Carl H. Norris in U.S. Pat. No. 3,877,818, owned by the United States of America. This technique, developed at the U.S.D.A. Agricultural Research Service in Greenbelt, Maryland, utilizes an instrument wherein a meat sample is exposed to infrared radiation from an incandescent light source. The radiation is transmitted through or reflected from the meat sample onto a tilting mirror which causes the respective transmitted or reflected light from the meat to pass through a planar interference filter at varying angles of incidence. Varying the angle of incidence of the filter by oscillating the tilting mirror produces a corresponding change in the wavelength of the radiation passing through the filter over a narrow bandwidth in the infrared spectrum. A photodetector receives the light transmitted through the filter and generates an electrical signal that is processed to read the fat content of the sample.
The present invention improves upon these and other prior art optical analyzers by providing a novel apparatus for tilting a set of interference filters in order to achieve greater variations in the wavelength of light transmitted through each filter.